The Force is the energy that flows through the cosmic ecosystem. It is the energy between all things, a tension, a balance that binds the universe together.
If everything in the universe is connected - every atom of every living thing and the space between them - then the force can literally travel anywhere, in any direction. As an energy with no mass, it should take no "time" to "travel" anywhere.
However, your specific question involves interaction with physical beings. The force user attacking would have some thinking and reaction time. Even though the force may take no time to "travel" to its target, this would give the other party the same amount of reaction time.
Let's face it, in real life, nobody has reactions fast enough to dodge a speeding bullet. If anybody is able to react and avoid being hit, their reaction was to their assailant drawing and raising the weapon. If they waited until the trigger had been pulled to react, they would have little chance.
Although force-users in SW can invoke the force silently and without movement, in the heat of battle we do see that they have primitive reactions - they raise their arms and appear to try and physically manipulate the force with hand movements. As these are obviously not necessary, such movements appear to be instinctive and it is these actions that could give someone time to react, if anything.
Addressing the "projectile" aspect of the question - I don't believe that fighting with the force can be like shooting projectiles of energy when that energy is already everywhere surrounding everything. However, just as the force of combustion inside the chamber of a gun forces the bullet to shoot out, perhaps the kind of "projectile" shooting seen in some Star Wars stories is actually matter - air, even - which is now a projectile because of the way that the force has interacted with it? So it isn't force energy that travels from person to person, but rather the attacker uses the force to make whatever form of matter is between the two persons move like a projectile.